Supramolecular Motifs in the Crystal Structures of Triethylbenzene Derivatives Bearing Pyridinium Subunits in Combination with Pyrimidinyl or Pyridinyl Groups

A series of mono- and dicationic 1,3,5-trisubstituted 2,4,6-triethylbenzenes containing pyridinium groups in combination with aminopyrimidine-/aminopyridine-based recognition units were synthesized and crystallographically studied. The combination of neutral and ionic building blocks represents a promising strategy for the development of effective and selective artificial receptors for anionic substrates. In the crystalline state, the investigated compounds show a tendency to bind the counterion PF6− in the cavity formed by the three functionalized side-arms. The intermolecular interactions with the PF6− ion comprise N-H∙∙∙F and C-H∙∙∙F bonds. Detailed analysis of various supramolecular motifs, including interactions with solvent molecules, provides deeper insights into the processes of molecular recognition. The information obtained is useful in the development of new receptor molecules for anions and in the selection of the most appropriate counterion.


Introduction
The extensive studies performed by our group to develop artificial carbohydrate receptors have clearly demonstrated that the consideration of different receptor building blocks capable of forming multiple noncovalent interactions is a promising strategy to achieve the effective and selective recognition of carbohydrates [1][2][3][4][5][6][7][8].The use of such a recognition strategy, based on combined noncovalent interactions, is known from carbohydrate-binding proteins [9,10] and served as a source of inspiration for studies with artificial systems.
In the development of new receptor molecules, the combination of different neutral functional groups, as well as of neutral and ionic residues, has been considered.As examples of receptors bearing both neutral and ionic recognition groups, the representatives of 1,3,5-substituted 2,4,6-triethylbenzene derivatives can be mentioned, [5] which were developed for the recognition of the anionic carbohydrate N-acetylneuraminic acid (Neu5Ac), the most abundant sialic acid (for a discussion on the importance of the detection of free sialic acids in biological samples, see [11]).These receptor molecules contain cationic pyridinium/quinolinium groups in combination with neutral moieties, such as aminopyridine-based recognition groups, and are able to complex Neu5Ac in highly competitive aqueous media.The building blocks of these receptor molecules enable Neu5Ac binding through a combination of neutral/charge-reinforced hydrogen bonds, ion pairs, CH-π interactions and van der Waals forces.In contrast, the identically substituted triethylbenzene derivatives with three cationic recognition sites are unable to effectively complex Neu5Ac in aqueous media but showed strong binding in less competitive solvents, such as acetonitrile, as expected.At this point, it should be mentioned that studies with triethylbenzene-based compounds possessing three cationic groups, such as pyridinium, bipyridinium, quinolinium or imidazolium groups, have made a very important contribution to the field of anion recognition [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].
In this work, we report the syntheses and crystal structures of new representatives of 1,3,5-substituted 2,4,6-triethylbenzene derivatives containing pyridinium groups (3-methylpyridinium, 3-hydroxypyridinium or 2-amino-5-methylpyridinium) in combination with aminopyrimidine-based recognition moieties.These new compounds are analogues of the aforementioned Neu5Ac receptor molecules with aminopyridine-based recognition groups.Their molecular architecture offers numerous possibilities for structural variations, enabling the synthesis of a whole range of compounds for systematic binding studies.It should be noted that recognition strategies for anionic substrates based on combined noncovalent interactions have been very well summarized in a review article [28] (for examples of reviews on anion recognition, see refs.[29][30][31][32][33][34]).
In this work, we report the syntheses and crystal structures of new representatives of 1,3,5-substituted 2,4,6-triethylbenzene derivatives containing pyridinium groups (3methylpyridinium, 3-hydroxypyridinium or 2-amino-5-methylpyridinium) in combination with aminopyrimidine-based recognition moieties.These new compounds are analogues of the aforementioned Neu5Ac receptor molecules with aminopyridinebased recognition groups.Their molecular architecture offers numerous possibilities for structural variations, enabling the synthesis of a whole range of compounds for systematic binding studies.It should be noted that recognition strategies for anionic substrates based on combined noncovalent interactions have been very well summarized in a review article [28] (for examples of reviews on anion recognition, see refs.[29][30][31][32][33][34]).
Here, the crystal structures of salts of mono-and dicationic triethylbenzene derivatives 1a-6a with PF6 − as counterions are described, comprising the solvent-free crystal structure of 1a and the solvates 2a•EtOH (2S), 3a•MeOH•H2O (2:1:3; 3S), 4a•H2O (4S), 5a•EtOH (5S) and 6a•CHCl3 (6S) (see Figure 1).In addition to the crystal structures of the new compounds 2a, 3a, 5a and 6a with pyrimidinyl groups, the crystal structures of two compounds containing pyridinyl groups (compounds 1a and 4a) are also considered for comparative purposes.Detailed analysis of the various supramolecular motifs observed in the crystal structures of the hexafluorophosphate salts 1a-6a, including motifs involving solvent molecules, provides interesting insights into molecular recognition phenomena, making the obtained results valuable, among others, for the development of new artificial receptor molecules.Detailed analysis of the various supramolecular motifs observed in the crystal structures of the hexafluorophosphate salts 1a-6a, including motifs involving solvent molecules, provides interesting insights into molecular recognition phenomena, making the obtained results valuable, among others, for the development of new artificial receptor molecules.
The bromide salts were converted to the corresponding hexafluorophosphates 1a-6a by treatment with an excess of sodium hexafluorophosphate in water or methanol.

Crystallographic Studies
Crystals suitable for X-ray diffraction analysis were grown by the slow evaporation of the solvent from solutions of the respective compounds.In the cases of 1a, 3a, 4a and 6a, the corresponding compound was suspended in water and mixed with small amounts of methanol, chloroform or acetonitrile until a clear solution was formed.Crystal growth then occurred by the slow evaporation of the organic solvent, yielding the solvent-free structure 1a (see Figure 2) as well as the solvates 3a•MeOH•H 2 O (2:1:3; 3S), 4a•H 2 O (4S) and 6a•CHCl 3 (6S).Crystals of compounds 2a and 5a were obtained from ethanol as solvates 2a•EtOH (2S) and 5a•EtOH (5S), respectively.In the case of 3S, two complexes 3S-

Crystallographic Studies
Crystals suitable for X-ray diffraction analysis were grown by the slow evaporation of the solvent from solutions of the respective compounds.In the cases of 1a, 3a, 4a and 6a, the corresponding compound was suspended in water and mixed with small amounts of methanol, chloroform or acetonitrile until a clear solution was formed.Crystal growth then occurred by the slow evaporation of the organic solvent, yielding the solvent-free structure 1a (see Figure 2) as well as the solvates 3a•MeOH•H2O (2:1:3; 3S), 4a•H2O (4S) and 6a•CHCl3 (6S).Crystals of compounds 2a and 5a were obtained from ethanol as solvates 2a•EtOH (2S) and 5a•EtOH (5S), respectively.In the case of 3S, two complexes 3S-I (3 + PF6 − •H2O) and 3S-II [3 + PF6 − •(H2O)2•CH3OH] were observed, as shown in Figure 2. Crystallographic data and the selected refinement parameters of the crystal structures are provided in Table S1.The geometric features of the receptors are described by the calculation of dihedral angles between the aromatic rings, designated as A-D in the figures showing the molecular structures.Their values, together with relevant torsion Crystallographic data and the selected refinement parameters of the crystal structures are provided in Table S1.The geometric features of the receptors are described by the calculation of dihedral angles between the aromatic rings, designated as A-D in the figures showing the molecular structures.Their values, together with relevant torsion angles, are presented in Table S2, while information regarding the non-covalent molecular interactions in the crystals is given in Table S3.O; 2:1:3), the receptor cations reveal common structural features showing an alternating arrangement of the substituents above and below the plane of the central arene ring, which is the frequently observed conformation of molecules based on the 1,3,5-triethylbenzene platform [37][38][39].Taking the ethyl groups into account, the spatial arrangement of the substituents along the periphery of the benzene ring represents an ab ab ab pattern (a = above, b = below, a /b = ethyl; for details, see references [40,41]).
In the solvent-free structure 1a (space group P-1, Z = 2), which is illustrated in Figures 2a, 3a  angles, are presented in Table S2, while information regarding the non-covalent molecular interactions in the crystals is given in Table S3.
In the solvent-free structure 1a (space group P-1, Z = 2), which is illustrated in Figures 2a-4a, the PF6 − ion resides in the receptor cavity created by the functionalized side-arms bearing heterocyclic groups.The two pyridinylaminomethyl moieties of the receptor display a significant twist, which is reflected by torsion angles of −156.Hexafluorophosphate salt 2a crystallizes from ethanol as colorless rods of the space group P-1 with the asymmetric part of the unit cell containing one receptor cation, one PF 6 − ion disordered over three positions and one solvent molecule (solvate 2S).The ORTEP diagram and a ball-and-stick representation of the complex 2 + PF 6 − •EtOH are shown in Figures 3b and 4b, respectively.The structure of the receptor-anion unit 2 + PF 6 − is similar to that of 1 + PF 6 − .Also, in the present case, the PF 6 − ion is connected to the receptor by N-H  Hexafluorophosphate salt 2a crystallizes from ethanol as colorless rods of the space group P-1 with the asymmetric part of the unit cell containing one receptor cation, one PF6 − ion disordered over three positions and one solvent molecule (solvate 2S).The ORTEP diagram and a ball-and-stick representation of the complex 2 + PF6 − •EtOH are shown in Figures 3b and 4b, respectively.The structure of the receptor-anion unit 2 + PF6 − is similar to that of 1 + PF6 − .Also, in the present case, the PF6 − ion is connected to the receptor by N-H The receptor cation of the hexafluorophosphate salt 3a differs from the aforementioned case by the presence of an OH group in the meta-position of the The receptor cation of the hexafluorophosphate salt 3a differs from the aforementioned case by the presence of an OH group in the meta-position of the pyridinium ring.This structural difference has a fundamental influence on the packing and coordination behavior of the molecules in the solid-state structure.The crystal growth of 3a from methanol and water The two independent cations in the asymmetric unit of 3S have slightly different conformations.These differences are expressed in Figure S1 (see Supporting Information), showing the superposition of the two molecules.
The solvent molecules, as well as the hydroxy group of the receptor, connect the complexes to one-dimensional structure domains that extend parallel to the crystallographic a-axis (Figure 7a).These aggregates are characterized by structurally different supramolecular ring motifs (see Figure 7b), which follow the graph sets R 3 2 (8) and R 3 3 (8).The two independent cations in the asymmetric unit of 3S have slightly different conformations.These differences are expressed in Figure S1 (see Supporting Information), showing the superposition of the two molecules.
The solvent molecules, as well as the hydroxy group of the receptor, connect the complexes to one-dimensional structure domains that extend parallel to the crystallographic a-axis (Figure 7a).These aggregates are characterized by structurally different supramolecular ring motifs (see Figure 7b), which follow the graph sets R3 2 (8) and R3 3 (8).

Hexafluorophosphate Salts of the Dicationic Compounds: Solvates 4S, 5S and 6S
The crystals of 4a obtained from acetonitrile and water (7:1, v/v) proved to be a mono-hydrate 4a•H 2 O [solvate 4S; space group P-1] with the asymmetric unit of the cell containing the dicationic receptor, two PF 6 − ions and one molecule of water, which form a complex of the structure, shown in Figures 2d, 8a         Crystal growth of the salt 5a from ethanol yields a solvate 5a•EtOH (5S) of the space group P-1 with the receptor cation, two PF 6 − ions, one of them disordered over two positions, and one twofold disordered ethanol molecule in the asymmetric part of the unit cell.The three functionalized side-arms of the receptor, two of which bear a 3-methylpyridinium group and one a dimethylpyrimidinylamino moiety, are arranged on the same face of the central benzene ring (ab ab ab conformation).The aromatic rings of these units are inclined at angles of 51.8(1), 78.9(1) and 36.The colorless crystals of 6a obtained from CHCl 3 also proved to be a solvate 6a•CHCl 3 [solvate 6S; space group P-1] with one molecule of the dicationic receptor, two disordered PF 6 − counterions and one solvent molecule in the asymmetric unit of the cell [6 2+ (PF 6 − ) 2 •CHCl 3 ].In the crystal, the receptor does not possess a preorganized binding pocket, as it displays a "two-up, one-down" arrangement of the functionalized arms.As is obvious from Figures 8c and 9c, the two 2-amino-5-methylpyridinium fragments are located on opposite sides of the central arene ring with their amino groups pointing away from the central benzene ring.Taking into account the ethyl groups, the receptor adopts an ab'ab'ba' conformation.The PF 6 − ions participate in different ways in complex formation.One of them (anion 1 in Figure 9c) is connected to the receptor by a N amino -H

Conclusions
Crystal structures of the hexafluorophospate salts 1a-6a, bearing pyridinium-and aminopyridine-or aminopyrimidine-based groups, show a tendency of these molecules to bind the PF 6 − counterion in the cavity formed by the three functionalized side-arms (see Figure 2).The intermolecular interactions comprise N-H•••F and C-H•••F bonds, the latter involving mostly an ortho-H atom of the pyridinium group.
In the case of the monocationic compounds 1a-3a, the only exception is represented by one of the two complexes observed in the crystal structure of the solvate 3S (see Figure 2c), where the cavity of the receptor cation is occupied by one methanol and two water molecules [3 ] represents an interesting finding.It is worth noting that the presence of two types of complexes was also observed by us in the crystal structures formed by acyclic receptors and glucopyranosides [50,51], as well as for ammonium receptors [52].
Among the dicationic compounds 4a-6a, the molecular cation 6 2+ does not possess a preorganized binding pocket (Figure 2f), because the two pyridinium groups project in opposite directions with respect to the central benzene ring (the molecule adopts an ab ab ba conformation; a = above, b = below, a /b = ethyl).In all other cases (1a, 2S, 3S-I, 3S-II, 4S and 5S) the arrangement of the substituents around the benzene ring follows an ab ab ab pattern.
The replacement of the pyridinyl (compound 1a and 4a) by pyrimidinyl group(s) (compound 2a and 5a, respectively) does not cause any significant changes in terms of the conformation and interactions of the receptor cation with the counterion (see 1a vs. 2S and 4S vs. 5S).
The obtained results suggest that, for receptor molecules of the type studied, other counterions, e.g., tetraphenylborate, should also be considered in molecular recognition studies of anionic substrates.
The detailed analysis of the various supramolecular motifs observed in the crystal structures of the hexafluorophosphate salts 1a-6a, including those involving solvent molecules, provides valuable information on the interactions of the receptor cation with the counterion, gives deeper insights into the processes of molecular recognition, and is helpful in the development of new receptor molecules for anions and in the selection of the most suitable counterion.

X-ray Crystallography
The intensity data of 2S was collected on a Smart APEX II diffractometer (Bruker AXS) with MoK α radiation (λ = 0.71073 Å) using ω-and φ-scans.Data integration and reduction were processed with SAINT-NT [53].An empirical absorption correction was applied to the collected reflections with SADABS [53].Preliminary structure models were derived by the application of direct methods [54] and were refined by a full-matrix least-squares calculation based on F 2 for all reflections.The intensity data of 1a, 3S, 4S, 5S and 6S were collected on a STOE IPDS 2T diffractometer with MoK α radiation (λ = 0.71073 Å) using the rotation method.The data reduction was processed with X-RED [55].An empirical absorption correction was applied to the collected reflections with STOE X-SHAPE [55].Preliminary structure models were derived by the application of direct methods using SIR2014 [56].The non-hydrogen atoms of the cationic receptors were refined with anisotropic thermal parameters.The atoms of the highly disordered anions in 1a, 2S and 3S were refined isotropically.With the exception of amino hydrogens, the OH hydrogens of the alcohol and water molecules in 2S, 3S, 4S and 5S, all other hydrogen atoms were included in the models in calculated positions and were refined as constrained to bonding atoms.
and 4a, the PF 6 − ion resides in the receptor cavity created by the functionalized side-arms bearing heterocyclic groups.The two pyridinylaminomethyl moieties of the receptor display a significant twist, which is reflected by torsion angles of −156.5(3) and −170.3(3)• for their atomic sequences C benzene -C-N-C pyridine .The pyridine rings (B, C) of these substituents are inclined at angles of 75.5(1) and 81.8(1) • with respect to the plane of the central arene ring (A), whereas the pyridinium group (D) is oriented nearly perpendicular (89.5(1) • ) to this plane.The interactions between the receptor cation and the counter ion comprise N-H•••F bonds [d(H•••F) 2.29(6)-2.58(4)Å] and a relatively short C-H•••F bond [d(H•••F) 2.37 Å], the latter involving an ortho-H atom of the pyridinium group.It should be mentioned that the participation of the fluorine atom of PF 6 -in the formation of hydrogen bonds with a variety of X-H donors [X = C, N, O.; X-H•••F(P) interaction] was discussed in reference [42] (for a review on hydrogen bonds with organic fluorine, including C-H•••F(C), N-H•••F(C) and O-H•••F(C) bonds, see reference [43]).Molecules 2023, 28, x FOR PEER REVIEW 5 of 19 5(3) and −170.3(3)° for their atomic sequences Cbenzene-C-N-Cpyridine.The pyridine rings (B, C) of these substituents are inclined at angles of 75.5(1) and 81.8(1)° with respect to the plane of the central arene ring (A), whereas the pyridinium group (D) is oriented nearly perpendicular (89.5(1)°) to this plane.The interactions between the receptor cation and the counter ion comprise N-H•••F bonds [d(H•••F) 2.29(6)-2.58(4)Å] and a relatively short C-H•••F bond [d(H•••F) 2.37 Å], the latter involving an ortho-H atom of the pyridinium group.It should be mentioned that the participation of the fluorine atom of PF6 -in the formation of hydrogen bonds with a variety of X-H donors [X = C, N, O.; X-H•••F(P) interaction] was discussed in reference [42] (for a review on hydrogen bonds with organic fluorine, including C-H•••F(C), N-H•••F(C) and O-H•••F(C) bonds, see reference [43]).

Figure 4 .
Figure 4. Ball-and-stick representations (side views) of the molecular structures of 1a (a), 2a•EtOH (2S) (b) and 3a•MeOH•H2O (2:1:3; 3S) (c), including the labeling of coordinating atoms.For the sake of clarity, only one component of the disordered anions is shown.As is obvious from Figure5, the crystal of 1a is composed of dimers, in which two ion pairs 1 + PF6 − are connected by C-H•••F bonds involving the para-H atom of the pyridinium ring.These dimers are further linked by offset π•••π interactions[44,45] with a centroid-centroid distance of 3.862(2) Å between the interacting pyridine rings.

Figure 4 . 19 Figure 3 .
Figure 4. Ball-and-stick representations (side views) of the molecular structures of 1a (a), 2a•EtOH (2S) (b) and 3a•MeOH•H 2 O (2:1:3; 3S) (c), including the labeling of coordinating atoms.For the sake of clarity, only one component of the disordered anions is shown.As is obvious from Figure 5, the crystal of 1a is composed of dimers, in which two ion pairs 1 + PF 6 − are connected by C-H•••F bonds involving the para-H atom of the pyridinium ring.These dimers are further linked by offset π•••π interactions [44,45] with a centroidcentroid distance of 3.862(2) Å between the interacting pyridine rings.

Figure 4 .
Figure 4. Ball-and-stick representations (side views) of the molecular structures of 1a (a), 2a•EtOH (2S) (b) and 3a•MeOH•H2O (2:1:3; 3S) (c), including the labeling of coordinating atoms.For the sake of clarity, only one component of the disordered anions is shown.As is obvious from Figure5, the crystal of 1a is composed of dimers, in which two ion pairs 1 + PF6 − are connected by C-H•••F bonds involving the para-H atom of the pyridinium ring.These dimers are further linked by offset π•••π interactions[44,45] with a centroid-centroid distance of 3.862(2) Å between the interacting pyridine rings.

Figure 6 .
Figure 6.(a) Excerpt of the crystal structure of the complex 2a•EtOH (2S) and (b) structure motif including the labeling of coordinating atoms.Only the major disorder position of the PF6 − ion is shown for clarity.Dashed lines represent hydrogen bond interactions.In (b), the 16-membered ring motif is marked by color highlighting.Nitrogen atoms are displayed as blue, oxygen as red, phosphorus as violet and fluorine atoms as green circles.In (b), the following interactions are shown: N-H•••F (a/b = 2.30, 2.46 Å), O-H•••N (c = 2.02 Å), C-H•••O (d = 2.57 Å) and C-H•••F (e/f = 2.35, 2.51 Å).

Figure 6 .
Figure 6.(a) Excerpt of the crystal structure of the complex 2a•EtOH (2S) and (b) structure motif including the labeling of coordinating atoms.Only the major disorder position of the PF 6 − ion is shown for clarity.Dashed lines represent hydrogen bond interactions.In (b), the 16-membered ring motif is marked by color highlighting.Nitrogen atoms are displayed as blue, oxygen as red, phosphorus as violet and fluorine atoms as green circles.In (b), the following interactions are shown: N-H•••F (a/b = 2.30, 2.46 Å), O-H•••N (c = 2.02 Å), C-H•••O (d = 2.57 Å) and C-H•••F (e/f = 2.35, 2.51 Å).
(1:1, v/v) yields colorless plates of the orthorhombic space group Pna2 1 with two receptor cations, two PF 6 − ions, one methanol molecule and three water molecules in the asymmetric unit of the cell [(3 + PF 6 − ) 2 •MeOH•(H 2 O) 3 ; solvate 3S].These components are combined to form two different complexes 3S-I (3 + PF 6 − •H 2 O) and 3S-II [3 + PF 6 − •(H 2 O) 2 •CH 3 OH], the structures of which are shown in Figures 2c, 3c and 4c.In the case of complex 3S-I, the geometry of 3 + PF 6 − and the mode of cation-anion interaction resemble that of 1 + PF 6 − and 2 + PF 6 − .The water molecule is connected by an O-H•••N bond [d(H•••N) 2.03 Å] to the pyrimidine-N-atom N4 of the receptor, while its second H atom acts as a trifurcated donor for O-H•••F bonding [d(H•••F) 2.28-2.51Å] to the PF 6 − ion.In the second complex (3S-II), the cavity of the receptor cation is occupied by one methanol and two water molecules, which participate in the formation of a pattern of N-H•••O, O-H•••O and O-H•••N bonds with the amine H atoms and a pyrimidine N atom of the receptor [d(H•••N) 1.91 Å, d(H•••O) 1.86-2.49Å].The PF 6 − ion is connected to a methyl H atom of the alcohol molecule by a weak C-H•••F bond [d(H•••F) 2.53 Å].
pyridinium ring.This structural difference has a fundamental influence on the packing and coordination behavior of the molecules in the solid-state structure.The crystal growth of 3a from methanol and water (1:1, v/v) yields colorless plates of the orthorhombic space group Pna21 with two receptor cations, two PF6 − ions, one methanol molecule and three water molecules in the asymmetric unit of the cell [(3 + PF6 − )2•MeOH•(H2O)3; solvate 3S].These components are combined to form two different complexes 3S-I (3 + PF6 − •H2O) and 3S-II [3 + PF6 − •(H2O)2•CH3OH], the structures of which are shown in Figures 2c-4c.In the case of complex 3S-I, the geometry of 3 + PF6 − and the mode of cation-anion interaction resemble that of 1 + PF6 − and 2 + PF6 − .The water molecule is connected by an O-H•••N bond [d(H•••N) 2.03 Å] to the pyrimidine-N-atom N4 of the receptor, while its second H atom acts as a trifurcated donor for O-H•••F bonding [d(H•••F) 2.28-2.51Å] to the PF6 − ion.In the second complex (3S-II), the cavity of the receptor cation is occupied by one methanol and two water molecules, which participate in the formation of a pattern of N-H•••O, O-H•••O and O-H•••N bonds with the amine H atoms and a pyrimidine N atom of the receptor [d(H•••N) 1.91 Å, d(H•••O) 1.86-2.49Å].The PF6 − ion is connected to a methyl H atom of the alcohol molecule by a weak C-H•••F bond [d(H•••F) 2.53 Å].
and 9a.The PF 6 − ion accommodated in the cavity of the receptor is disordered over two positions (s.o.f.0.85/0.15).The major disorder component of this anion interacts with the receptor by a N-H•••F [d(H•••F) 2.21(1) Å] and three C-H•••F bonds [d(H•••F) 2.39-2.55Å], whereas the minor disorder position of this anion is involved in the formation of four C-H•••F bonds [d(H•••F) 2.33-2.54Å].The hydrogen atoms of the water molecule act as bifurcated binding sites for O-H•••F bonds [d(H•••F) 2.05(2)-2.54(3)Å] with the anions of the complex.

Figure 8 .
Figure 8. ORTEP diagrams of the molecular structures of the complexes 4a•H2O (4S) (a), 5a•EtOH (5S) (b) and 6a•CHCl3 (6S) (c), including atom labeling and ring specification.Displacement ellipsoids are drawn at the 50% probability level.Dashed lines represent hydrogen bond interactions.For the sake of clarity, only the major disorder positions of the PF6 − ions in 5S and 6S are shown.

Figure 8 . 19 Figure 9 .
Figure 8. ORTEP diagrams of the molecular structures of the complexes 4a•H 2 O (4S) (a), 5a•EtOH (5S) (b) and 6a•CHCl 3 (6S) (c), including atom labeling and ring specification.Displacement ellipsoids are drawn at the 50% probability level.Dashed lines represent hydrogen bond interactions.For the sake of clarity, only the major disorder positions of the PF 6 − ions in 5S and 6S are shown.Molecules 2023, 28, x FOR PEER REVIEW 10 of 19

( 1 )
• to one another, indicating a less symmetric conformation.The PF 6 − ion located inside the cavity of the receptor cation is connected by one N-H•••F [d(H•••F) 2.29(2) Å] and three C-H•••F hydrogen bonds [d(H•••F) 2.24-2.48Å], the latter involving H atoms of the pyridinium groups.The second PF 6 − ion is located outside the cavity.The two disorder positions of the alcohol molecule contribute in a different way to complex formation.In one case, its OH hydrogen atom is connected to the pyrimidine nitrogen N2 of the receptor [d(H•••N) 2.49 Å].In the second disorder position of the solvent, the OH hydrogen is linked to the PF 6 − ion (anion 2 in Figure 9b) via O-H•••F bonding [d(H•••F) 2.36 Å].A variety of C-H•••N [d(H•••N) 2.38-2.60Å] and C-H•••F hydrogen bonds [d(H•••F) 2.43-2.55Å] (see Figure 11 ) connect the complexes to a close three-dimensional network.The two disorder positions of the alcohol molecule contribute in a different way to complex formation.In one case, its OH hydrogen atom is connected to the pyrimidine nitrogen N2 of the receptor [d(H•••N) 2.49 Å].In the second disorder position of the solvent, the OH hydrogen is linked to the PF6 − ion (anion 2 in Figure9b) via O-H•••F bonding [d(H•••F) 2.36 Å].A variety of C-H•••N [d(H•••N) 2.38-2.60Å] and C-H•••F hydrogen bonds [d(H•••F) 2.43-2.55Å] (see Figure11) connect the complexes to a close threedimensional network.

Figure 11 .
Figure 11.Packing diagram of the complex 5a•EtOH (5b) viewed down the crystallographic a-axis.Dashed lines represent hydrogen bond interactions.Nitrogen atoms are displayed as blue, oxygen as red, phosphorus as violet and fluorine atoms as green circles.For the sake of clarity, only one position of the disordered anion/solvent is shown.The following interactions are shown: N-H•••F (a = 2.29 Å), O-H•••N (b = 2.49 Å) and C-H•••F (c-h = 2.30-2.54Å).The colorless crystals of 6a obtained from CHCl3 also proved to be a solvate 6a•CHCl3 [solvate 6S; space group P-1] with one molecule of the dicationic receptor, two disordered PF6 − counterions and one solvent molecule in the asymmetric unit of the cell [6 2+ (PF6 − )2•CHCl3].In the crystal, the receptor does not possess a preorganized binding

Figure 11 .
Figure 11.Packing diagram of the complex 5a•EtOH (5S) viewed down the crystallographic a-axis.Dashed lines represent hydrogen bond interactions.Nitrogen atoms are displayed as blue, oxygen as red, phosphorus as violet and fluorine atoms as green circles.For the sake of clarity, only one position of the disordered anion/solvent is shown.The following interactions are shown: N-H•••F (a = 2.29 Å), O-H•••N (b = 2.49 Å) and C-H•••F (c-h = 2.30-2.54Å).
26(2) Å] as well as C-H•••F interactions.The major disorder component of the second anion is linked to the amino hydrogen H5A [d(H•••F) 2.41 Å] and the bifurcated donor atom H16B [d(H•••F) 2.48, 2.54 Å].The solvent molecule is associated by a C-H•••N bond with the pyrimidine-N atom N3 of the receptor [d(H•••N) 2.63 Å].Viewing the crystal structure as in Figure 12 reveals that the complex molecules are connected to double strandlike supramolecular aggregates extending parallel to the crystallographic b-axis.Molecules of adjacent strands are connected by offset π•••π (face-to-face) interactions with a Cg•••Cg distance of 3.629(3) Å and a slippage of 0.858 Å between the interacting pyrimidine rings.donor atom H16B [d(H•••F) 2.48, 2.54 Å].The solvent molecule is associated by a C-H•••N bond with the pyrimidine-N atom N3 of the receptor [d(H•••N) 2.63 Å].Viewing the crystal structure as in Figure 12 reveals that the complex molecules are connected to double strand-like supramolecular aggregates extending parallel to the crystallographic b-axis.Molecules of adjacent strands are connected by offset π•••π (face-to-face) interactions with a Cg•••Cg distance of 3.629(3) Å and a slippage of 0.858 Å between the interacting pyrimidine rings.